1. Field of the Invention
The present invention relates to a manufacturing method for a continuously cast product of steel and, more particularly, to a method for controlling casting conditions by detecting the position of a solidification end (referred to as a crater end) of a cast product.
2. Description of Related Arts
In continuous casting of steel, in order to improve the productivity and quality of product, it is very important that the position of a crater end of cast product in the casting direction be detected, and casting conditions be controlled based on the detection result.
For example, if the casting speed is increased to improve the productivity, the crater end moves to the downstream side in the casting direction. However, if the crater end moves beyond a region in which product support rolls lie, the product is swelled by static pressure (referred to as bulging), which presents a problem of degraded quality or casting stop due to large bulging. Also, in the case where soft reduction is applied to a product to decrease central segregation of product and thereby achieve high quality, it is necessary to control the casting speed and the quantity of secondary cooling water so that the crater end is positioned in a soft reduction zone. Further, in the case where the position of crater end is varied greatly in the casting direction by the change of casting conditions, a product on the upstream side of crater end in the casting direction solidifies first, and the supply of molten steel to the downstream side is stopped, so that porosity or laminar voids are created in the central portion of product, which causes a defect that greatly decreases the yield of final product. Also, in the case where the position of crater end varies greatly in the casting direction, even if the casting speed and the quantity of secondary cooling water are controlled, it is difficult to induce the crater end to the soft reduction zone.
In order to detect the position of crater end, it is necessary to continuously measure the solidification state of product. Various methods for this measurement have been proposed so far. Among these methods, many methods in which the transverse waves of ultrasonic waves (hereinafter referred to as transverse ultrasonic waves) are utilized have been proposed. This is because since the transverse ultrasonic wave has a property such that it propagates in a solid phase only and does not propagate in a liquid phase, if the transverse ultrasonic waves are transmitted in the thickness direction at a position of product and a signal indicating that the transverse ultrasonic waves have propagated in the product is detected, it can be judged that the position has been solidified completely, and if the signal is not obtained, it can be judged that unsolidified layer remains. Also, there is available a method in which the position of crater end is estimated from time of flight in which the transverse ultrasonic waves propagates in a product.
As a method for generating the transverse ultrasonic waves in a hot product and detecting them, an electromagnetic ultrasonic wave method in which ultrasonic waves are transmitted and received electromagnetically is known. As a method for measuring the solidification state of product by using the electromagnetic ultrasonic wave method, a method in which a product is held between two transverse ultrasonic wave sensors and the signal intensity of transverse ultrasonic waves having propagated in the product is measured has been disclosed in JP-A-52-130422.
JP-A-62-148850 discloses a method in which an electromagnetic ultrasonic wave sensor capable of generating longitudinal waves and transverse waves at the same time is used to measure the solidification state by the signal intensity of transverse ultrasonic waves, and the variations in liftoff (gap between product and sensor) and the abnormality of sensor are checked at the same time by additionally using a signal of longitudinal ultrasonic waves propagating in an unsolidified layer.
JP-A-10-197502 discloses a method in which a resonance frequency of transverse ultrasonic waves in a product is measured, and a solid phase ratio (ratio of solid phase to solid-liquid coexistence phase) is determined from this resonance frequency.
However, in these methods for measuring the solidification state of product by using electromagnetic ultrasonic waves, the sensitivity is low and the S/N (signal-to-noise ratio) is also low, so that sufficient measurement accuracy cannot be obtained. Also, for this reason, the liftoff of electromagnetic ultrasonic wave sensor is inevitably decreased to about 2 mm, so that continuous measurement cannot be made stably for a long time.
To solve the problem, for example, JP-A-11-183449 discloses a method in which a touch roll is installed on the sensor and the touch roll is pushed against a product to make continuous measurement for a long time. In this method, however, if the sensor is used continuously in an environment in which the temperature exceeds several hundred degrees Centigrade and much scale exists, the scale gets stuck between the sensor and the product, so that the sensor may be damaged or the touch roll sticks to the product, which makes continuous measurement difficult.
Therefore, it is necessary that the liftoff be widened by increasing the sensitivity of electromagnetic ultrasonic wave sensor and that the measurement be done in a non-contact manner without the use of touch roll.
As a method for increasing the sensitivity of electromagnetic ultrasonic wave sensor, JP-A-53-106085 discloses a method in which electromagnetic ultrasonic waves by Lorentz's force is used and a cooling fluid is blown to a hot steel to decrease the temperature of steel to a temperature not higher than the Curie point, by which the steel is magnetized and the electric conductivity is increased. In this method, since the driving force F of electromagnetic ultrasonic waves by Lorentz's force is expressed by F=B×J by using magnetic flux density B and electric current density J, as B and J increase, the sensitivity is made higher.
JP-A-2000-266730 discloses a method in which a burst-like transmitting signal in which at least one selected from frequency, amplitude, and phase is modulated within a predetermined pulse width is used, and correlation operation of receiving signal is performed by using a reference signal of a waveform that is the same as or similar to the transmitting signal. In this method, the correlation between receiving signal and transmitting signal is high, and the correlation between noise and transmitting signal is low, so that the S/N is increased by the correlation operation.
JP-A-53-57088 discloses a method in which receiving signals are averaged in synchronization with an electromagnetic ultrasonic wave generator. In this method, noise has a random waveform generated for each pulse repetition, so that the S/N is increased by averaging.
However, in the method described in JP-A-53-106085, the magnetic flux density of steel is low near the Curie point, so that the steel must be cooled rapidly to a temperature range 200° C. or more lower than the Curie point to obtain a high magnetic flux density, which impairs the quality of product. Also, the conversion efficiency of electromagnetic ultrasonic waves by Lorentz's force is very low essentially, so that an effect of increasing the S/N is little.
If electromagnetic ultrasonic waves are applied to the method described in JP-A-2000-266730, since the receiving signal is far weaker than the transmitting signal and the transmitting signal leaks into the receiving signal, if the pulse width of burst wave is too long, the transmitting signal hides the receiving signal. In particular, if this method is applied to continuous casting, the inside temperature of product changes during the operation, and the position at which the receiving signal appears varies, so that the pulse width cannot be made too long, and an effect of increasing the S/N is little.
If the method described in JP-A-53-57088 is applied to continuous casting, the position at which the receiving signal appears varies as described above, so that the average number must be increased. Therefore, an effect of increasing the S/N is little.
As described above, in the prior art, since the S/N cannot be increased sufficiently, the liftoff of electromagnetic ultrasonic wave sensor cannot be increased, so that the position of crater end cannot be detected stably and exactly in a non-contact state.